I highly recommend pearl gouramis. It's fairly easy to tell the boys from the girls if they've reached adolescense. The boys will have a much oranger/redder throat, while the girls are more silvery. I had a male-female pair for several months in a 29-gallon, which turned out to be too small a tank (at least for my pair). They're very graceful, calm fish. You might be better off getting two females for one male. I also like moonlight gouramis, but I've never had them before. I do know that they can eventually grow to 5 - 6 inches -- a truly beautiful shimmering sight, but with that potential size, you want to take care that you'll have enough swimming room in your tank once everyone reaches maturity. By the way, you're tanks are both lovely! Can't wait to see more pics after you've finished stocking.